Question

Find the sum of first 20 terms of A.P. 3, 7, 11, 15 .... using the formula​

Answer 1

Answer:

The given AP are:

3, 7, 11, 15, ........

Here, first term (a) = 3, common difference(d) = 7 - 3 = 4 and

The number of terms (n) = 20

To find, the sum of 20 terms of the given AP(S_{20}S

20

) = ?

We know that,

The sum of nth terms of the AP

S_{n}=\dfrac{n}{2} [2a+(n-1)d]S

n

=

2

n

[2a+(n−1)d]

The sum of 20 terms of the given AP.

S_{20}S

20

= \dfrac{20}{2} [2(3)+(20-1)4]

2

20

[2(3)+(20−1)4]

= 10(6 + 19 × 4)

= 10(6 + 76)

= 10(82)

= 820

∴ The sum of 20 terms of the given AP(S_{20}S

20

) = 820